Field of the Invention
This invention relates to designs for the reactant flow field plates employed in solid polymer electrolyte fuel cells, and particularly to those subject to ice blockages during freeze start-up.
Description of the Related Art
Fuel cells such as solid polymer electrolyte or proton exchange membrane fuel cells electrochemically convert reactants, namely fuel (such as hydrogen) and oxidant (such as oxygen or air), to generate electric power. Solid polymer electrolyte fuel cells generally employ a proton conducting, solid polymer membrane electrolyte between cathode and anode electrodes. A structure comprising a solid polymer membrane electrolyte sandwiched between these two electrodes is known as a membrane electrode assembly (MEA). In a typical fuel cell, flow field plates comprising numerous fluid distribution channels for the reactants are provided on either side of a MEA to distribute fuel and oxidant to the respective electrodes and to remove by-products of the electrochemical reactions taking place within the fuel cell. Water is the primary by-product in a cell operating on hydrogen and air reactants. Because the output voltage of a single cell is of order of 1V, a plurality of cells is usually stacked together in series for commercial applications in order to provide a higher output voltage. Fuel cell stacks can be further connected in arrays of interconnected stacks in series and/or parallel for use in automotive applications and the like.
Along with water, heat is a significant by-product from the electrochemical reactions taking place within the fuel cell. Means for cooling a fuel cell stack is thus generally required. Stacks designed to achieve high power density (e.g. automotive stacks) typically circulate liquid coolant throughout the stack in order to remove heat quickly and efficiently. To accomplish this, coolant flow fields comprising numerous coolant channels are also typically incorporated in the flow field plates of the cells in the stacks. The coolant flow fields may be formed on the electrochemically inactive surfaces of the flow field plates and thus can distribute coolant evenly throughout the cells while keeping the coolant reliably separated from the reactants. By appropriate design, a sealed coolant flow field is created when both fuel and oxidant side plates are mated together into a bipolar plate assembly. The sealed coolant flow field can thus serve to distribute coolant evenly throughout the cells while keeping the coolant reliably separated from the reactants.
To provide both reactants and the coolant to and from the individual cells in the stack, a series of outlets and inlets are generally provided at opposing ends of the individual cells such that when the cells are stacked together they form manifolds for these fluids. Further required design features then are passageways in the plates to distribute the bulk fluids in these formed manifolds to and from the various channels in the reactant and coolant flow fields in the plates. Herein, these passageway regions are referred to as the transition regions. The transition regions can themselves comprise numerous fluid distribution channels, e.g. oxidant and/or fuel transition channels.
Another desirable feature in the flow field plates can include the use of what are known in the art as backfeed ports. Such ports allow for bulk fluids to initially be distributed from the formed manifolds to the “back” or inactive sides of the flow field plates and then subsequently to be fed to the active side of the plates through the backfeed ports. A reactant backfeed port is thus fluidly connected to a manifold inlet or outlet for that reactant via some suitable passage formed in the coolant surface of the plate. And the reactant backfeed port is also fluidly connected to the reactant flow field on the reactant surface of the plate via the passageways of the associated transition region.
US2008/0311461 for instance discloses exemplary flow field plate constructions incorporating backfeed features. Further, US2008/0311461 discloses flow field plate embodiments in which the various inlets and outlets forming the reactant manifolds are located on the sides of the flow field plates (i.e. these reactant inlets and outlets are not in line with the flow fields themselves). This configuration of the inlets and/or outlets is known in the art as a sidefeed configuration.
Accumulations of liquid water can undesirably occur for various reasons in various locations within the flow field plates during operation of solid polymer electrolyte fuel cells. And various designs have been developed to address problems caused by such accumulation in the art. JP2004207039 for instance discloses an approach to make the flow of reactant gas possible when a passage in the flow field of a fuel cell is blocked with water drops. Here, recessed groove-shaped gas passages are installed in the flow field to serve as bypass passages so that when other passages are blocked by water drops, reactant gas can instead bypass through a gas diffusion layer.
In fuel cell stacks subject to freezing temperatures, accumulations of liquid water can be additionally problematic because, when the water freezes, the ice formed can undesirably block fluid flows or the associated expansion of the solid ice can cause damage to the cell. Significant sized accumulations of liquid water which may be subject to freezing are therefore generally avoided, either by preventing accumulation in the first place or alternatively by removing them before they have the opportunity to freeze. For example, the approach employed in US2008/0113254 attempts to prevent undesirable water accumulation by use of an appropriate porous medium located between the plates. Alternatively, various techniques are disclosed in the art for removing water from a fuel cell stack prior to shutdown and storage in subzero temperatures.
In other approaches, the aim may be to temporarily tolerate the formation of ice in the plates and address the issues associated with that in other ways. As an example, in US20130089802, flow field plate constructions are disclosed for use in fuel cell stacks that are subject to freezing temperatures. In designs having internal coolant flow fields and reactant backfeed ports, relief ducts are provided in the supporting walls surrounding the backfeed ports in order to allow for ice formation and thus prevent cracking of the plates.
There remains a need however for more options and improvements in addressing problems associated with ice formation in flow field plates of fuel cells subject to below freezing temperatures. This invention fulfills these needs and provides further related advantages.